1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor laser and, more particularly, to a method of manufacturing a semiconductor laser for a light source of an optical disc, having a preferable characteristic at the time of high-frequency superposition. In addition, the present invention relates to a structure and a method of manufacturing a semiconductor laser having a window structure at its facet and capable of high-output operation.
2. Description of the Prior Art
A prior art short-wavelength InGaAlP visible light laser diode is reported in, "Short-Wavelength InGaAsP visible Light Laser Diode" IEEE JOURNAL OF QUANTUM ELECTRONICS. VOL. 27, NO.6, JUNE 1991, Gen-ichi Takahashi et.al. pages 1476-1482.
The contents of this publication will be quoted in order to describe the prior art short-wavelength InGaAlP visible light laser diode.
Considerable progress has been made in the development of short-wavelength InGaAlP visible-light laser diodes. This paper describes the recent results achieved on the operation of these InGaAlP lasers. It is found that there is a strong correlation between operation temperature and oscillation wavelength. The deterioration of favorable temperature characteristics in the short-wavelength region is attributed to an increase in the leakage current, which originates in a small conduction-band discontinuity inherent in the InGaAlP material system. The introduction of a highly doped p-cladding layer has a remarkable effect on the improvement of these temperature characteristics. Short-wavelength oscillation at 630 nm band wavelength was achieved for transverse-mode stabilized InGaAlP lasers.
I. INTRODUCTION
InGaAlP visible-light laser diodes oscillating in the 0.6 .mu.m wavelength range have attracted much interest as light sources for optical information processing systems, such as high-density optical disk systems and high-speed laser printers. Room-temperature continuous-wave (CW) operation of InGaAlP lasers was first achieved in 1985 for InGaP ternary active layers at a wavelength of 670-690 nm. Since then, much effort has been expended on the improvement of laser performance, such as transverse-mode stabilization, threshold-current reduction, high-temperature operation, and high reliability.
Among these laser diode characteristics, high-power and short-wavelength operation are important subjects for the practical use of InGaAlP lasers. High-power operation is required especially for optical disk applications. Fundamental methods, previously used to obtain high-power operation for conventional GaAlAs lasers, are applicable to the InGaAlP systems. High-power CW operation, as high as 50 mW, has been achieved for a transverse-mode stabilized InGaAlP laser by using a thin active-layer structure and antireflection and high-reflection coatings on the laser facets. A high-power broad stripe laser, with 320 mW output power, was also reported. There has also been a remarkable improvement in the reliability of high-power InGaAlP lasers. Transverse-mode stabilized InGaAlP lasers with 20 mW output power are now commercially available. The topical achievement in 1990 for high-power InGaAlP lasers was the realization of a window-structure laser. By using window structures, output powers were increased to 80 and 75 mW for the InGaAlP lasers with gain-guided and transverse-mode stabilized structures, respectively.
The development of shorter-wavelength InGaAlP laser diodes, oscillating below 670 nm, has been continued for use in high-density optical disk applications and the realization of a small-size light source of relevant emission wavelength for the substitution of conventional HeNe gas lasers (.lambda.=633 nm). Three methods were investigated for obtaining short-wavelength InGaAlP laser diodes: 1) employment of InGaAlP quaternary active layer, 2) quantum-well structure, and 3) utilization of the bandgap energy dependence on the substrate orientation. The first method is a relatively simple, but fundamental, approach for shortening the oscillation wavelength.
The problem in realizing a short-wavelength InGaAlP laser diode is that it is difficult to obtain high-temperature operation. The maximum operation temperature decreases with decreasing wavelength. For practical use of short-wavelength InGaAlP lasers, improvement of the temperature characteristics is indispensable. The deterioration of the temperature characteristics in the short-wavelength region is thought to originate from an increase in a leakage current caused by a small bandgap energy difference between the active and the p-type cladding layers. The leakage current is reduced by using a large bandgap cladding layer or by using a highly doped p-type cladding layer. The bandgap energy of the cladding layer has a limit determined by an InGaAlP material system which is lattice matched to the GaAs substrate. Therefore, it was found necessary to employ a highly doped cladding layer for realizing short-wavelength InGaAlP laser diodes. Recently, a crystal growth technique, which yields a highly doped p-type cladding layer, has been developed. This effectively improved the temperature characteristics of InGaAlP laser diodes. Due to this improvement, 630 nm band CW operation at room temperature has been realized.
This paper reviews our recent results on the short-wavelength operation of InGaAlP visible-light laser diodes. The transverse-mode stabilized structures used for the short-wavelength InGaAlP lasers are briefly shown. Theoretical background for the reduction of the leakage current and experimental results of short-wavelength InGaAlP laser diodes will be described.
II. DEVICE STRUCTURE FOR TRANSVERSE-MODE STABILIZED InGaAlP LASER DIODES
For the realization of short-wavelength InGaAlP laser diodes, employment of a transverse-mode stabilized structure is necessary for operation-current reduction, as well as for the practical application of the device. FIG. 14 shows a typical configuration for the transverse-mode stabilized InGaAlP laser. This structure was fabricated by a three-step metalorganic chemical vapor deposition (MOCVD) method, including the n-GaAs layer growth, utilizing a selective growth on the ridge-shaped double-heterostructure. So the structure was called a selectively buried ridge waveguide (SBR) laser.
The optical-mode confinement in the SBR laser is obtained by using a loss-guide effect, which is produced by a complex refractive-index distribution along the junction plane. In such a ridge-stripe structure, the effective refractive-index difference .DELTA.N.sub.eff between the stripe and outside region is an improvement factor for the optical characteristics of the output beam. .DELTA.N.sub.eff is determined by the refractive indexes of each layer, active-layer thickness d, and distance h between the active layer and the n-GaAs absorbing layer in the region outside the stripe. FIG. 15 shows a calculated example of .DELTA.N.sub.eff as a function of d and h for the case of InGaP active layer and In.sub.0.5 (Ga.sub.0.3 Al.sub.0.7).sub.0.5 P cladding layers. The preferable value of .DELTA.N.sub.eff, for realizing stable fundamental-mode oscillation and low-astigmatism characteristic, is about 10.sup.-2. The parameters d and h should be optimized to satisfy this requirement.
FIG. 16 shows another structure for a transverse-mode stabilized InGaAlP laser. In this laser, current confinement is realized by utilizing a heterobarrier caused by a large band discontinuity between the p-InGaAlP cladding layer and p-GaAs contact layer. The laser is called a heterobarrier-blocking (HBB) laser. The principle of the current confining mechanism for the HBB laser is shown in FIG. 17. Material parameters, used in the calculation for FIG. 17 and other figures shown later, are listed in FIG. 26. A characteristic feature of the InGaAlP material is that the valence band discontinuity .DELTA.E.sub.v between InGaAlP and GaAs is very large. This band discontinuity causes a large spike in the valence-band profile at the interface of p-InGaAlP and GaAs, which acts as a barrier for the holes. When the Al composition ratio, of the p-InGaAlP, is relatively large, current does not flow through the p-InGaAlP-p-GaAs interface, as shown in FIG. 17(a). Conversely, the valence-band spike is reduced by introducing a p-InGaP layer which has an intermediate bandgap energy between InGaAlP and GaAs. Therefore, current easily flows in the structure shown in FIG. 17(b), which corresponds to the configuration of the stripe region of the HBB laser.
The optical characteristics of the SBR and the HBB lasers are almost identical, because the optical confinement mechanism is essentially the same. In regard to the fabrication process, the HBB laser is simpler to grow than the SBR laser because the HBB structure requires only a two-step MOCVD growth process.
These transverse-mode stabilized structures, shown in FIGS. 14 and 16, were used for the production of short-wavelength InGaAlP lasers. The characteristics of these lasers will be shown later.
III. THEORETICAL BACKGROUND FOR SHORT-WAVELENGTH OPERATION
Difficulties in shortening the lasing wavelength of the InGaAlP laser diode arise from electron leakage from the active layer to the p-type cladding layer. FIG. 18 shows a schematic band diagram for the InGaAlP double-heterostructure laser at an oscillation threshold. In the InGaAlP double-heterostructure, the conduction-band discontinuity and the bandgap difference between the active and the cladding layers, are not so large. For example the conduction-band discontinuity value for the InGaP-In.sub.0.5 (Ga.sub.0.3 Al.sub.0.7).sub.0.5 P double-heterostructure, which has the largest bandgap difference within the extent of direct-transition region, is 0.19 eV. Due to this small value for the conduction-band heterobarrier, a part of the injected current overflows from the active layer to the p-type cladding layer, as shown in FIG. 18.
The effect of the electron overflow is seen to a much greater extent during high-temperature operation, because the barrier height .delta..sub.p (see FIG. 18) decreases due to the increase in the quasi-Fermi level .phi..sub.n in the active layer caused by a high-injection current at the higher temperature. FIG. 19 shows a calculated example of the temperature dependence of threshold-current density for the InGaP-In.sub.0.5 (Ga.sub.0.3 Al.sub.0.7).sub.0.5 P double-heterostructure. The total threshold current is represented as a summation of net injection current into the active layer and the leakage current J.sub.L. The net injection current density shows a temperature dependence with a characteristic temperature of 200 K. The apparent characteristic temperature at room temperature is lower than this value due to the existence of the leakage current. Most of this leakage current is attributed to the electron overflow current, which flows over the heterobarrier at the conduction band between the active and the p-type cladding layers.
The electron overflow becomes more and more conspicuous for shorter-wavelength laser diodes. This is attributed to the decrease in the heterobarrier height caused by the reduction in bandgap energy difference between the active and the cladding layers. Therefore, it is difficult to realize high-temperature operation in the short-wavelength region.
The electron overflow is reduced by increasing the barrier height between the quasi-Fermi level in the conduction band of the active layer and the bottom of the conduction band of the p-type cladding layer. As shown in FIG. 18, the barrier height .delta..sub.p is determined by the bandgap energy difference between the active and the p-type cladding layers, and the Fermi level, E.sub.fp, of the p-type cladding layer. The Fermi level E.sub.fp is determined by the hole concentration in the cladding layer. Therefore, .delta..sub.p is increased by using either a large bandgap or a highly doped cladding layer.
FIG. 20 shows numerical simulation results for the threshold-current density as a function of active-layer bandgap. The drastic increase in the threshold current in the shorter-wavelength region is attributed to electron overflow. Experimental data for the laser diodes, having an In.sub.0.5 (Ga.sub.0.3 Al.sub.0.7).sub.0.5 P cladding layer with a hole concentration of 4.times.10.sup.17 cm.sup.-3 showed a similar tendency as the corresponding curve in FIG. 20. However, the increase in the threshold current occurred over a longer wavelength region than that shown by the calculated results. This difference is considered to originate from an assumption in the calculation, whereby the nonradiative recombination lifetime, for the active layer, was assumed to be equal to that for InGaP and independent of the Al composition.
As shown in FIG. 20, the threshold current, in shorter-wavelength region devices, is reduced by using a p-type cladding layer with a high Al composition ratio, which has a large bandgap energy, and a high hole concentration. Although the bandgap of InAlP (x=1) is indirect and the conduction band offset is smaller than that for In.sub.0.5 (Ga.sub.0.3 Al.sub.0.7).sub.0.5 P, it still has the effect of reducing the electron overflow. This is because the barrier height .delta..sub.p, in FIG. 18, is determined mainly by E.sup.clad and E.sub.fp.
IV. EXPERIMENTAL RESULTS
As indicated above, a higher Al composition ratio, and a higher hole concentration for p-type cladding layer, is preferable for the development of short-wavelength InGaAlP laser diodes. However, it is difficult to obtain a highly doped p-type cladding layer, especially for high-Al-composition InGaAlP. Therefore, it is necessary to choose an optimum Al composition ratio for the cladding layer. In the experiment, an Al composition ratio of 0.7, a point at which the InGaAlP has its largest direct bandgap, was selected. For the case of Zn p-type doping, a limiting factor exists; that is, at high concentrations doping causes Zn diffusion into the active layer, which results in a degradation of device characteristics. The maximum doping level, without Zn diffusion into the active layer, depends on the growth conditions, for example, growth temperature. Zn doping characteristics were investigated in detail for InGaAlP double-heterostructures. It was found that this diffusion into the active layer does not occur for acceptor concentration levels below 6.times.10.sup.17 cm.sup.-3, under optimized growth conditions. The use of such a highly doped p-type cladding layer effectively improved the temperature characteristics of the short-wavelength InGaAlP laser diodes. Due to this improvement, a high-temperature short-wavelength oscillation device was obtained for a laser employing an InGaAlP quaternary active layer.
FIG. 21 shows the experimental results representing the relationship between the oscillation wavelength and the maximum temperature for continuous-wave operation of InGaAlP lasers. As shown in the figure, there is a strong correlation between maximum operation temperature T.sub.max and the wavelength .lambda.. The shorter the wavelength is, the lower the possible operation temperature becomes. This is considered to originate in the electron overflow as described above. The effect of the highly doped p-type cladding layer, predicted in FIG. 20, was also confirmed by experiment. As shown in FIG. 21, there was a remarkable improvement for T.sub.max when the acceptor concentration was 5.times.10.sup.17 cm.sup.-3.
By using the highly doped p-type cladding layer (p=5.times.10.sup.17 cm.sup.-3), the maximum CW temperature is increased to 70.degree. C. for a laser with an oscillation wavelength of 650 nm, as shown in FIG. 21. FIG. 22 shows the characteristic temperature of the 650 nm wavelength SBR lasers plotted against acceptor concentration. A higher characteristic temperature was obtained by employing a higher acceptor concentration. FIG. 23 shows an example of the aging test results for uncoated 650 nm lasers at a heat-sink temperature of 50.degree. C. The lasers operated for more than 1000 h.
For realizing a 630 nm band InGaAlP laser, the Al composition ratio for the active layer was set as 0.15. The SBR structure with this quaternary active layer, and In.sub.0.5 (Ga.sub.0.3 Al.sub.0.7).sub.0.5 P cladding layers, was fabricated. FIG. 24 shows light output versus current characteristics for this 630-nm-band SBR laser. Room-temperature CW operation at a wavelength of 638 nm was achieved. The threshold current at 25.degree. C. was 100 mA and slope efficiency was 0.35 W/A/facet. Continuous-wave operation was maintained, even at 50.degree. C. This improvement in the operation temperature is considered to be the effect of the leakage-current reduction brought about by the high doping technique for the p-cladding layer.
Short-wavelength operation was also achieved by using the HBB structure. FIG. 25 shows light output versus current characteristics for the 630 nm band HBB laser. A single longitudinal mode oscillation at a wavelength of 636.1 nm was obtained. The beam divergence angles in the direction parallel and perpendicular to the junction plane were 8.5.degree. and 39.degree., respectively.
Hereinafter, a specific structure, a function, and an operation of a conventional 0.67 .mu.m band InGaP/InAlGaP semiconductor laser will be described.
FIG. 11 is a schematic view of a conventional 0.67 .mu.m band InGaP/InAlGaP semiconductor laser which is the same as the laser described alone, and FIGS. 12(a)-12(h) are sectional views showing manufacturing steps of the semiconductor laser. In FIG. 11, an n-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 2 having an impurity concentration of 1.times.10.sup.17 cm.sup.-3 and a thickness of 1.5 .mu.m is disposed on an n-GaAs semiconductor substrate 1 having an impurity concentration of 1 to 3.times.10.sup.18 cm.sup.-3 and a thickness of 95 .mu.m. An undoped In.sub.0.5 Ga.sub.0.5 P active layer 3 having a thickness of 0.07 .mu.m is disposed on the n-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 2. A p-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P upper cladding layer 4 having an impurity concentration of 5.times.10.sup.17 (the impurity is Zn or Si) and a thickness of 0.25 .mu.m is disposed on the In.sub.0.5 Ga.sub.0.5 P active layer 3. A p-GaAs first contact layer 5 having an impurity concentration of 2.times.10.sup.19 cm.sup.-3 and a thickness of 0.4 .mu.m is disposed on a ridge 4a of the p-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P upper cladding layer 4. The width of the ridge is 5.5 .mu.m at the bottom and 3.0 .mu.m at the top surface. An n-GaAs current blocking layer 9 having an impurity concentration of 6.times.10.sup.18 cm.sup.-3 and a thickness of 1 .mu.m is disposed on a thin portion 4b of the p-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P upper cladding layer 4. A p-GaAs second contact layer 11 having an impurity concentration of 2.times.10.sup.19 cm.sup.-3 and a thickness of 2.5 .mu.m is disposed on the p-GaAs first contact layer 5 and on the n-GaAs current blocking layer 9. In addition, a p-side electrode 12 comprising Ti/Pt/Au is disposed on the p-GaAs second contact layer 11, and an n-side electrode 13 comprising AuGe/Ni/Ti/Au is disposed on the p-GaAs semiconductor substrate 1. The height of the laser except the electrodes is 100 .mu.m.
In FIGS. 12(a)-12(h), a selective growth mask 16 comprising SiO.sub.2 or Si.sub.3 N.sub.4 is formed on the p-GaAs first contact layer 5, and a stripe-shaped selective growth mask 16a is formed by patterning the selective growth mask 16. A ridge region 8 comprising the p-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 4 and the p-GaAs first contact layer 5 is formed by etching, using the patterned stripe-shaped selective growth mask 16a.
Next, a manufacturing method of the conventional semiconductor laser will be described referring to FIGS. 12(a)-12(h).
First, the n-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 2, the In.sub.0.5 Ga.sub.0.5 P active layer 3, the p-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 4, and the p-GaAs first contact layer 5 are epitaxially grown on the n-GaAs semiconductor substrate 1 by an MOCVD (Metal Organic Chemical Vapor Deposition) method at 750.degree. C. FIG. 12(a) shows a sectional view of a grown wafer.
Then, the selective growth mask 16 comprising SiO.sub.2 or Si.sub.3 N.sub.4 is patterned so as to form the stripe-shaped film 16a as shown in FIG. 12(c). The width of the stripe is appropriately 5 to 15 .mu.m.
The selective growth mask 16a serves as a mask for ridge etching. More specifically, as shown in FIG. 12(d), etching is performed so as to produce the ridge 8 using the selective growth mask 16a. The etching is performed into the p-GaAs first contact layer 5 and into the middle of the p-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 4 (leaving a thickness of 0.3 .mu.m) to form the ridge 8. FIG. 12(d) shows a sectional view after the etching.
Then, as shown in FIG. 12(e), crystal growth is performed again and the part other than the ridge 8 is buried with the n-GaAs current blocking layer 9 having an impurity concentration of 6.times.10.sup.18 cm.sup.-3 and a thickness of 1 .mu.m. At this time, since the selective growth mask 16a exists at the ridge 8, there is no crystal growth thereon. During the crystal growth, the wafer is heated at 500.degree. C. or more (for example, 550.degree. C.) for approximately 30 minutes.
Then, as shown in FIG. 12(f), the selective growth mask 16a is etched away by wet or dry etching and then, the p-GaAs second contact layer 11 is further grown on the ridge 8 and on the n-GaAs current blocking layer 9.
Alternatively, in some cases, after the state shown in FIG. 12(e), the-ridge etching mask 16a is removed to form the state shown in FIG. 12(g) and, in this state, the impurity diffusion source film 7 comprising ZnO or the like is formed on the ridge 8 again. Then, p type impurities in the impurity diffusion source film 7 are diffused into the first contact layer 5 and the p type upper cladding layer 4 by heat treatment to form the p type diffused region 10. Then, the impurity diffusion source film 7 is removed and the second contact layer 11 is grown on the whole surface of the wafer.
Then, the n-side electrode 13 comprising AuGe/Ni/Ti/Au is formed on the n-GaAs semiconductor substrate 1 and the p-side electrode 12 comprising Ti/Pt/Au is formed on the p-GaAs second contact layer 11 using vapor deposition by resistance heating, EB (electron beam) deposition, or sputtering. Alternatively, after the step of FIG. 12(g), the electrodes are formed as described above. Thus, the conventional semiconductor laser shown in FIG. 11 is completed.
Next, function and operation of the conventional example will be described.
First, an operation of the conventional 0.67 .mu.m band semiconductor laser shown in FIG. 11 will be described.
When a voltage is applied so as to be positive on the p-side electrode and be negative on the n-side electrode, holes are injected into the In.sub.0.5 Ga.sub.0.5 P active layer 3 through the p-GaAs second contact layer 11, the p-GaAs first contact layer 5, and the p-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 4, and electrons are injected into the In.sub.0.5 Ga.sub.0.5 P active layer 3 through the n-GaAs semiconductor substrate 1 and the n-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 2. Consequently, the holes are recombined with the electrons and induced emission light is generated in the active layer 3. When the carriers are injected in a sufficiently large amount and light is generated beyond its loss in the waveguide of the active layer 3, laser oscillation occurs.
Then, the structure of the ridge will be described. According to the ridge structure having the ridge region 8 comprising the p-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 4 and the p-GaAs first contact layer 5, a pn junction is formed between the p-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 4 and the n-GaAs current blocking layer 9 and between the p-GaAs second contact layer 11 and the n-GaAs current blocking layer 9 at a region covered with the n-GaAs current blocking layer 9 other than the stripe-shaped ridge region 8. Therefore, even if a voltage is applied so that the p-electrode 12 becomes positive, since a pnp structure is formed between the p-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 4 and the p-GaAs second contact layer 11 at the region other than the ridge region 8, that is, there is a reverse bias, and a current does not flow. More specifically, since the n-GaAs current blocking layer 9 blocks the current literally, a current flows only in the ridge region 8. Therefore, the current is concentrated in the region of the active layer 3 in the vicinity of the ridge, whereby a current density sufficient for laser oscillation is attained. As a result, laser oscillation occurs.
In addition, the n-GaAs current blocking layer 9 has a property of absorbing the laser light generated in the active layer 3 because the band gap energy of GaAs is smaller than the band gap energy of In.sub.0.5 Ga.sub.0.5 P constituting the active layer 3. Therefore, since the laser light is strongly absorbed on both sides of the ridge 8, the laser light is also concentrated in the region in the vicinity of the ridge 8. As a result, a horizontal transverse-mode, which is an important characteristic in the stable operation of the semiconductor laser, has a configuration of a single-ridge.
As can be seen from the contents of the above quoted publication, although the laser characteristic of the semiconductor laser comprising InGaAlP system materials is improved by increasing the impurity concentration of the p type cladding layer, when a normal doping technique is used, it is difficult to increase the p type impurity concentration of the InGaAlP cladding layer, and an effective method for improving the p type impurity concentration of the p type cladding layer is not particularly specified in the above publication.
In addition, according to an AlGaAs or InGaP/InAlGaP system semiconductor laser which emits laser light in a band of 0.8 .mu.m or 0.67 .mu.m, used as a light source of an optical disc device, such as a compact disc (CD), the maximum optical output is decided by an optical output at which facet destruction occurs. The facet destruction occurs when the laser beam is absorbed by a surface level in a facet region. Therefore, in order to realize a high optical output operation, it is necessary to devise something so as not to generate the facet destruction even at the time of high-optical output. In this respect, it is very effective to provide a structure that prevents the laser beam from being absorbed at the facet region, that is, a window structure which is transparent to the laser beam.
The window structure to prevent the facet destruction in the semiconductor laser will be described hereinafter.
Catastrophic optical damage (COD) generated at the time of high-output operation of the semiconductor laser is caused by the band gap of the facet portion is smaller than that of the central portion even in the active layer of the same composition because of an interface level existing in the vicinity of the facet of the laser. More specifically, a temperature is partially raised at the facet because of absorption of laser light in the vicinity of the facet at the time of the high-output operation of the laser. Since the partial excessive heat generation further reduces the band gap at that part, absorption of laser light is accelerated so that the heat generation is increased, which becomes a feedback loop. At last, the laser facet is melted and irreversible destruction occurs. The laser output when the facet destruction occurs is called COD optical output, and the COD optical output restricts the maximum optical output of the semiconductor laser comprising AlGaAs or AlGaInP system materials.
In order to prevent the above-described facet destruction, the band gap of the active layer at a region where the interface level could be produced is made larger than that of the active layer at another region. For example, in Conference on Applied Physics in 1990, spring (Proceedings 29a-SA-7), there is disclosed a AlGaInP system semiconductor laser whose COD optical output is significantly increased to be high-powered by a window structure in which a forbidden band width of the active layer in the vicinity of the laser facet is wider than that of the active layer in the center of the laser.
It is known that when GaInP or AlGaInP crystal material is grown under a predetermined growth condition, a natural superlattice in which component atoms are periodically arranged is formed as a phenomenon peculiar to that material. In addition, it is known that when an impurity such as Zn is introduced into crystals having the natural superlattice structure to disorder the superlattice structure, a forbidden band width at the disordered region is larger than that of the region which is not disordered. In the above publication, the window structure is formed in such a manner that impurities are introduced in the vicinity of the laser facet after the growth of the active layer comprising GaInP or AlGaInP under a condition in which the natural superlattice is produced.
FIG. 27 is a schematic view showing a section of the conventional AlGaInP system semiconductor laser having the window structure in the longitudinal direction of a resonator. In FIG. 27, an n type AlGaInP lower cladding layer 102 is disposed on an n type GaAs substrate 101, a GaInP quantum-well (QW) active layer 103 having a natural superlattice structure is disposed on the lower cladding layer 102, a p type AlGaInP upper cladding layer 104 is disposed on the active layer 103, and a p type GaAs contact layer 105 is disposed on the upper cladding layer 104. In addition, an n-side electrode 106 is disposed on the n type GaAs substrate and a p-side electrode 107 is disposed on the p type GaAs contact layer 105. A Zn diffused region 108 is formed in the vicinity of the laser facet and a region 109 is formed in the active layer 103 by disordering the natural superlattice through Zn diffusion. In addition, laser beam 120 is emitted.
FIGS. 29(a)-29(d) show process steps of the manufacturing method of the AlGaInP system semiconductor laser having the window structure shown in FIG. 27. In the figures, the same references designate the same or corresponding parts in FIG. 27. The manufacturing steps of the semiconductor laser shown in FIG. 27 will be described referring to FIGS. 29(a)-29(d).
First, the n type AlGaInP lower cladding layer 102, the AlGaInP quantum-well active layer 103 and the p type AlGaInP upper cladding layer 104 are sequentially grown on the n type GaAs substrate 101, preferably by MOCVD. Then, the p type GaAs contact layer 105 is grown on the upper cladding layer 104, whereby a laser laminated structure shown in FIG. 29(a) is formed. At this time, its growth condition is controlled so that the crystal structure of the active layer 103 is the natural superlattice structure.
Then, as shown in FIG. 29(b), there is formed an SiO.sub.2 film pattern 110 having an opening 110a in the vicinity of a cleavage position shown by a two-dotted line in the figure. The width w of the opening 110a is approximately 20 .mu.m with regard to precision of the cleavage.
Then, as shown in FIG. 29(c), Zn atoms are diffused in the laser laminated structure by vapor-phase diffusion or solid-phase diffusion using the SiO.sub.2 film pattern 110 as a mask, to form the Zn diffused region 108. By the diffusion of Zn, the region in the vicinity of the laser facet of the active layer 103 becomes the region 109 in which the natural superlattice structure is disordered. The diffusion speed of Zn in GaAs is different from that in AlGaInP. Generally, it is higher in the AlGaInP quantum-well active layer 103 and the upper and lower AlGaInP cladding layers 104 and 102 than in the GaAs substrate 101 and the contact layer 105.
Then, the SiO.sub.2 film pattern 110 is removed and as shown in FIG. 29(d), the n-side electrode 106 is formed on the rear surface of the substrate 101, the p-side electrode 107 is formed on the contact layer 105, and a resonator facet 150 is formed by isolating elements by cleaving, whereby the semiconductor laser shown in FIG. 27 is completed.
Next, an operation thereof will be described. When a forward bias to the pn junction of the laser is applied to the n-side electrode 106 and the p-side electrode 107, electrons and holes are injected into the active layer 103 and recombined in the active layer 103 to emit light. The light generated in the active layer 103 is guided between a pair of resonator facets 150 along the active layer, and reflection and amplification are repeated to produce laser oscillation. At this time, the region in the vicinity of the resonator facet of the active layer 103 is the region 109 in which the natural superlattice structure is disordered by diffusion of Zn and the forbidden band width of the region 109 is larger than that of the active layer in the center of the laser in which the natural superlattice structure is not disordered. Consequently, according to the conventional example, absorption of light is prevented at the laser facet and, therefore, high-output laser operation becomes possible.
In addition, FIG. 28 is a schematic view showing a section of a conventional AlGaInP system semiconductor laser having another window structure in the longitudinal direction of the resonator. In FIG. 28, the same references as in FIG. 27 designate the same or corresponding parts, and a Zn diffused region 118 is formed in the vicinity of the laser facet by introducing impurities from the laser facet.
FIGS. 30(a)-30(d) are sectional views illustrating process steps in a manufacturing method of the AlGaInP system semiconductor laser having the window structure shown in FIG. 28. In the figures, the same references as in FIGS. 29(a)-29(d) designate the same or corresponding parts. The manufacturing steps of the semiconductor laser shown in FIG. 28 will be described referring to FIGS. 30(a)-30(d).
First, the n type AlGaInP lower cladding layer 102, the AlGaInP quantum-well active layer 103, and the p type AlGaInP upper cladding layer 104 are sequentially grown on the n type GaAs substrate 101, preferably by MOCVD. Then, the p type GaAs contact layer 105 is grown on the upper cladding layer 104, whereby the laser laminated structure shown in FIG. 30(a) is formed. At this time, growth conditions are controlled so that the crystal structure of the active layer 103 has the natural superlattice structure.
Then, as shown in FIG. 30(b), the laser laminated structure is cleaved at a position shown by the two-dotted line in FIG. 30(a) to form the resonator facet 150.
Then, Zn atoms are diffused from the resonator facet 150 into the laser laminated structure by vapor-phase diffusion or solid-phase diffusion, whereby the Zn diffused region 108 is formed as shown in FIG. 30(c). By the diffusion of Zn, the region in the vicinity of the laser facet of the active layer 103 becomes the region 109 in which the natural superlattice structure is disordered. At this time, the depth of the impurity diffusion is set so that the length d of the disordered region 109 (length of the window region) is 4 to 5 .mu.m.
Then, as shown in FIG. 30(d), the n-side electrode 106 is formed on the rear surface of the substrate 101 and the p-side electrode 107 is formed on the contact layer 105, whereby the semiconductor laser shown in FIG. 28 is completed.
Next, an operation thereof will be described. When a forward bias to the pn junction of the laser is applied to the n-side electrode 106 and the p-side electrode 107, electrons and holes are injected into the active layer 103 and recombined in the active layer 103 to emit light. The light generated in the active layer 103 is guided between a pair of resonator facets 150 along the active layer 103, and reflection and amplification are repeated to produce laser oscillation. The region in the vicinity of the resonator facet of the active layer 103 is the region 109 in which the natural superlattice structure is disordered by diffusion of Zn, and the forbidden band width of the region 109 is larger than that of the active layer in the center of the laser in which the natural superlattice structure is not disordered. Consequently, according to the conventional example, similar to the conventional example shown in FIG. 27, absorption of light is prevented at the laser facet and, therefore, high-output laser operation becomes possible. In addition, according to the conventional example, when the impurities are directly introduced from the resonator facet, the length of the window region in which the impurities are diffused is shorter than that in the conventional example shown in FIG. 27, whereby loss of absorption in the window region because of the impurities are reduced.
Similar to the above description, according to a semiconductor laser having an active layer which is 200 .ANG. or less in thickness and a quantum-well structure comprising a plurality of quantum-well layers and a plurality of barrier layers, an impurity, such as zinc (Zn) or silicon (Si), is diffused in the quantum-well structure and atoms constituting the well layers and barrier layers are mixed and disordered, whereby an effective bandgap energy of the quantum-well structure is equalized with that of the barrier layer and made higher than the effective bandgap energy of the quantum-well structure layer which is not disordered. Consequently, the window structure layer which is transparent to the laser light is provided at the disordered part, whereby the facet destruction at the time of high-output operation of the laser is prevented and the high-output operation becomes possible.
In addition, as described above, the In.sub.0.5 Ga.sub.0.5 P layer constituting the active layer has a quantum effect in which In and Ga are alternatingly laminated in a state where the crystal growth thereof is normally performed, which is called a natural superlattice. When the natural superlattice is disordered by Zn as described above, its bandgap energy is increased.
According to the above-described conventional InGaP/InAlGaP system semiconductor laser producing laser light in the 0.67 .mu.m band, the carrier concentration of the p-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 4 of the conventional semiconductor laser is approximately 8.times.10.sup.17 cm.sup.-3 at most. According to a doping method for the p-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 4 in the manufacturing steps of the conventional laser, gas of a dopant material (Zn, Si, or the like) is allowed to flow together with a raw material gas, such as Al, Ga, In, and P, which constitutes the layer, to be taken into the crystal. However, the doping amount during crystal growth is limited according to the kind of the crystal, and the p carrier concentration of the p-(Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer is approximately 8.times.10.sup.17 cm.sup.-3 at most. According to an experiment performed by inventors of the present invention, a carrier concentration for a ratio of DMZn flow rate to a III group raw material gas flow rate was approximately 5.times.10.sup.17 cm.sup.-3 at the highest as shown in FIG. 13 and the activation ratio of the carriers was limited to 90% at most. In addition, according to this experiment, the impurity concentration in the actual layer was 5.times.10.sup.17 cm.sup.-3 /0.9=5. 56.times.10.sup.17 cm.sup.-3 and it is supposed that the above value 8.times.10.sup.17 cm.sup.-3 is not included.
As described referring to FIGS. 12(g) and 12(h), it is possible that the ridge-etching mask 16a is removed to form the impurity diffusion source film 7 at the ridge after crystal growth and ridge etching, whereby the impurity diffusion is performed at the ridge. In this case, however, the number of steps is increased and it is necessary to align the impurity diffusion source film with the ridge when the impurity diffusion source film is formed. Consequently, the steps become complicated and its precision can not be high.
As a result, for example, as compared with the 0.78 .mu.m band AlGaAs/GaAs system semiconductor laser for an optical disc, the element resistance of the conventional 0.67 .mu.m band InGaP/InAlGaP system laser shown in FIG. 11 was increased. Although the semiconductor laser for the optical disc was conventionally used under a high-frequency superposition of 600 MHz, since the element resistance of the 0.67 .mu.m band semiconductor laser of the conventional example is high, only a high-frequency superposition of approximately 100 to 200 MHz is applied thereto, which is a problem in practical use.
In addition, since the conventional semiconductor laser having the window structure is manufactured by introducing the impurities to the facet after the crystal growth process is completed as described above, the manufacturing steps become complicated.